CIE Categories Archives: 3. Movement in and out of cells

3.3) Active transport

3.3) Active transport


Active transport: is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using the energy from respiration.


Examples of active transport include:

  • uptake of glucose by epithelial cells in the villi of the small intestine (and by kidney tubules)
  • uptake of ions from soil water by root hair cells in plants

Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.


  • Substance combines with carrier protein molecule


  • Carrier transports substances across membrane using energy from respiration
  • Substance released into cell

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3.2) Osmosis

3.2) Osmosis


Osmosis: is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentration solution) through a partially permeable membrane.


A dilute solution means it has lots of water molecules, and a high water potential.

A concentrated solution has few water molecules and low water potential.


  • Water moves in and out of cells by osmosis through the cell membrane.
  • It is important that the cells in an animal’s body are surrounded by a liquid which has the same concentration as the liquid inside the cells.
  • Tissue fluid: the liquid outside the cells.
  • Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall.

 If a plant cell is placed in distilled water, water molecules will move from the distilled water to the cell, the cell swells up and becomes turgid but it will never burst because plant cells are surrounded by cell walls, which are made of cellulose and is elastic, it will stretch but never break. The vacuole is exerting turgor pressure on the elastic cell wall.


If we place a plant cell in a concentrated salt solution with low water potential, water will move from the cell to the solution causing the cell to become plasmolysed.


The importance of osmosis:

Plants gain water through osmosis in their roots from the soil. Without a water potential gradient,water will be loss from the roots. Plant cells contain vacuoles, which, if not full with water, will cause the cell to become flaccid. If all the cells in a leaf become flaccid, the whole leaf will become flaccid, hence causing the plant to wilt. Plant cells therefore need water to remain turgid and keep firm.


If an animal cell surrounded with a high water potential, osmosis will take place, and if the water is not expelled some way or another, the cell will burst (a process called haemolysis in red blood cells). This is because an animal cell doesn’t have a cell wall to keep it strong. If an animal cell is surrounded with low water potential, the water in the cytoplasm will diffuse outwards, causing the cell to shrink (crenated)

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3.1) Diffusion

3.1) Diffusion

Diffusion: is the net movement of molecules and ions from a region of their higher concentration to a region of their lower concentration down a concentration gradient, as a result of their random movement.


Diffusion always takes place down a concentration gradient, that means that the particles that diffuse try to spread evenly in all spaces, so it moves from where it’s very concentrated to where it’s not concentrated.


Substances move into and out of cells by diffusion through the cell membrane.


The importance of diffusion of gases and solutes:



  • (Animals) Necessary for gas exchange in all living organisms (O2 in, CO2 out)
  • (Plants) Necessary for obtaining Carbon Dioxide and releasing oxygen during photosynthesis.


Solutes (liquids)

  • Dissolved salts diffuse through root hair cell.
  • Absorption of dissolved food material in many organisms, like amoeba, bacteria and fungi is carried out through diffusion.
  • Some digested food material is absorbed by diffusion


Water as a solvent

  • Plants cannot obtain minerals unless they are dissolved in water
  • Enzymes and hormones cannot be secreted unless they are dissolved in water
  • Excretory products cannot be excreted unless they are dissolved in water.


Rates of diffusion

The energy for diffusion comes from the kinetic energy of random movement of molecules and ions.

Factors that influenced the rate of diffusion:

  • Surface area – The larger the surface area of the exchange membrane the faster particles diffuse.
  • Temperature – Increasing the temperature will give particles more kinetic energy, making them move faster, thus increasing the rate of diffusion.
  • Concentration gradient – The steeper the gradient the faster the particles diffuse.
  • Distance (Thickness of exchange membrane) – The thinner it is, the easier it will be for particles to go through it, the faster the diffusion rate.
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